DESCRIPTION: The broad objectives of the parent proposal for this FIRCA are to understand microtubule dynamics in vivo and in vitro and, in particular, to determine the nature of the coupling between chromosome movement and microtubule growth and shortening. Specific aims targeted in the current grant period focus on analysis of microtubule formation, disassembly, turnover, and selective stabilization. Cell systems to be investigated include mammalian fibroblasts, mitotic cells, and neurons. This proposal addresses an apparent paradox in the MT dynamics field. Although dynamic instability is now established as a primary mechanism of MT turnover, consideration of the totality of evidence in vitro and in vivo suggests that dynamic instability, as originally formulated, cannot account for the rapidity of turnover in vivo. The specific hypothesis to be tested in this proposal is the "conveyor assembly" model for centrosome activity first proposed by the foreign collaborator in 1983. According to this view, the MT array is developed by initiation of MTs at the centrosome followed by their detachment (release) and centripetal displacement away from the centrosome. Two key predictions of the conveyor assembly model will be tested: (i) minus ends of MTs in cultured cells are rather stable; and (ii) MTs are continuously released from the centrosome.